A Spanish judge looking into the brutal repression of dictator General Francisco Franco's forces during the country's civil war has opened an investigation into the tens of thousands of victims of the so-called "red terror" unleashed by some of his leftwing opponents.

Judge Baltasar Garzón has asked authorities for a list of those killed on the orders of the tribunals set up by the leftwing Republican authorities in Madrid during the three-year civil war that ended in 1939.

The judge provoked controversy last month when he asked government departments and the Roman Catholic church to help provide a list of names of those placed before firing squads by Franco's military tribunals or killed by rightwing death squads.

That request followed a petition from campaigners who have been looking for and digging up the hundreds of mass graves containing the victims of Franco's forces and supporters. They delivered a list of more than 130,000 victims this week.

Garzón's initial investigation was criticised by rightwing politicians, who claimed he was stirring up old hatreds.

Conservative commentators also condemned his latest request yesterday. "It is a tactical move because there are doubts about whether the law permits Garzón to do what he is trying to do," historian José María Marco told the rightwing radio station Cope.

Campaigners said they saw no conflict. "If we are going to be rigorous we have to say that this is the best thing he can do," said Rafael Gil, a spokesman in Granada for one of the many groups digging up graves.

The names of those killed on the orders of Republican tribunals or by leftwing or anarchist death squads are readily available as they were gathered during Franco's 36-year dictatorship. Franco's courts dealt with many of the perpetrators of those atrocities - often by sending them to the firing squad.

A large white cross on a hill beside Madrid's Barajas airport marks the spot, at Paracuellos de Jarama, where several thousand victims of leftwing firing squads are buried.

Many campaigners say it is too late for the perpetrators of the repression carried out by Franco's side to be put on trial as the people involved are mostly dead and the crimes are covered by a 1977 amnesty law.